1. Field of the Invention
The present invention relates to a rolling bearing suitable for automobile transmission which exhibits a prolonged galling live even when operated lubricated at a high traction coefficient with foreign matters mixed therein. A rolling bearing, according to the present invention, exhibits a prolonged galling life even when used with an automobile engine auxiliary machinery which is possibly contaminated by water that makes it difficult to form a lubricant layer and exhibits a prolonged galling life even when operated lubricated at a high traction coefficient with foreign matters mixed and water therein. The present invention also relates to parts and bearings of toroidal stageless transmission which gives a high tangential force at high temperatures under a high load.
2. Description of the Related Art
In general, the inner race, outer race and rolling element constituting the conventional rolling bearing are obtained by forming a high carbon chromium steel (particularly SUJ2 steel) into a predetermined shape, and then hardening and tempering the material such that the surface hardness thereof and the amount of retained austenite are adjusted to about HRC62 and about 10 vol. %, respectively.
On the other hand, an automobile automatic transmission comprises torque converter, gear mechanism, hydraulic mechanism, wet clutch, etc. incorporated therein. In order to allow these mechanisms to operate smoothly for power transmission, the rolling bearing for automatic transmission uses a lubricant having a high traction coefficient (automatic transmission fluid: ATF). Accordingly, if the foregoing conventional rolling bearing is used, the tangential force developed between the race track surface of the bearing race and the rolling surface of the rolling element increases during the operation of the automatic transmission, making it easy for lubricant layer to be destroyed and hence causing early galling on the race track surface or rolling surface.
Further, the rolling bearing mounted on an automatic transmission is liable to contamination of the lubricant provided therein by metal cuttings, shavings, burr, abrasion powder, etc. When the foregoing conventional rolling bearing is operated lubricated with these foreign matters mixed therein, these foreign matters cause dents on the race track surface of the bearing race or the rolling surface of the rolling element. With these dents as starting points, flaking (scale-like galling) occurs, drastically reducing the life of the rolling bearing.
As a conventional technique for preventing early galling on the race track surface there is disclosed in JP-A-4-28845 (The term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined Japanese patent publication (kokai)xe2x80x9d) an approach involving the formation of a bearing race for rolling bearing form a steel material comprising as alloying components from 0.65 to 0.90 wt % of C, from 0.15 to 0.50 wt % of Si, from 0.15 to 1.0 wt % of Mn and 2.0 to 5.0 wt % of Cr. This technique is intended for a rolling bearing which operates subject to vibration or impact load due to high speed rotation. It is described that the cause of early galling on the race track surface in such an operating atmosphere is a mechanism where C is diffused in the material at the position where the maximum shearing stress is produced, and then fixed to form a hard and brittle white layer. It is claimed that the use of a steel material having the foregoing composition with a low C concentration and a high Cr concentration makes it difficult for such a white layer to be formed, preventing the occurrence of early galling on the race track surface.
As a conventional technique for prolonging the life of a rolling bearing operating lubricated with foreign matters mixed therein there is disclosed in JP-B-6-11899 (The term xe2x80x9cJP-Bxe2x80x9d as used herein means an xe2x80x9cexamined Japanese patent publicationxe2x80x9d) an approach which comprises subjecting a steel material comprising as alloying components from 0.4 to 0.8 wt % of C, from 0.3 to 1.2 wt % of Si, not more than 1.0 wt % of Mn, from 4.0 to 8.0 wt % of Cr and from 0.1 to 1.0 wt % of Mo as a bearing steel to carburizing or carbonitriding to form carbides or carbonitrides on the surface thereof at an area ratio of from 20% to 35%. It is claimed that in accordance with this technique, the definition of the formulation of the steel material to be used makes it possible to enhance the toughness and the resistance of the matrix to temper softening and causes finely particulate M23C6 or M7C3 type carbide or carbonitride to be dispersed and deposited in the surface layer to strengthen the surface layer, prolonging the life of the bearing race operating lubricated with foreign matters mixed therein.
However, such a rolling bearing operating lubricated at a high traction coefficient with foreign matters mixed therein as the foregoing rolling bearing for automobile automatic transmission cannot be expected to exhibit a prolonged galling life when the foregoing conventional technique is applied. The approach for prolonging the life of a rolling bearing operating lubricated with foreign matters mixed therein described in JP-B-6-11899 is disadvantageous in that the requirement for carburizing or carbonitriding, which adds to facility cost, raises the production cost.
Further, a rolling bearing operating lubricated at a high traction coefficient and high temperatures is liable to contamination of the lubricant by water drops which have been developed by the condensation of water vapor in the space around the bearing due to sudden temperature change in operation. Moreover, water which has been left in the automatic transmission after washing with water or the like can contaminate the lubricant. Such a rolling bearing operating lubricated at a high traction coefficient with water mixed therein is disadvantageous in that a lubricant layer can hardly be formed on the race track surface or rolling surface. The lubricant, if any, can be easily destroyed by the increased tangential force as mentioned above. Thus, such a rolling bearing cannot be expected to exhibit a prolonged galling life when the foregoing conventional technique is applied.
On the other hand, JP-B-7-110988 discloses as a rolling bearing having a prolonged life desirable for transmission or engine a rolling bearing comprising a bearing race and a rolling element at least one of which is made of an alloy steel comprising at least from 0.3 to 0.6 wt % of C and from 3 to 14 wt % of Cr and has a surface layer obtained by carburizing or carbonitriding the material and then subjecting the material to heat treatment so that it is hardened, the surface layer containing finely particulate carbides and retained austenite in an amount of from 20 to 50 vol. % and from 10 to 25 vol. %, respectively. The rolling bearing thus proposed exhibits a longer life than the conventional rolling bearing when operated lubricated with a turbine oil contaminated by foreign matters. However, this rolling bearing leaves something to be desired in the life shown when operated lubricated at a high traction coefficient with foreign matters and water mixed therein.
It is an object of the present invention to provide a rolling bearing which exhibits a prolonged galling life even when operated lubricated at a high traction coefficient with foreign matters mixed therein and lubricated at a high traction coefficient with water mixed therein.
According toe a first aspect of the present invention, a rolling bearing comprises an inner race, an outer race and a rolling element, in which at least one of the inner race, the outer race and the rolling element is obtained by forming from a steel material containing carbon (C) of 0.55 wt % to 1.10 wt % and chromium (Cr) of 2.0 wt % to 9.0 wt %, and then subjecting the material thus forming to hardening, tempering and polishing. In the rolling bearing, a chromium oxide (Cr2O3) layer having a thickness of 5 nm to 300 nm is provided on at least one of a race track surface of the inner race, a race track surface of the outer race and a rolling surface of the rolling element.
According to a second aspect of the present invention, in the rolling bearing according to the first aspect, an amount of austenite retained in at least one of the race track surface of the inner race, the race track surface of the outer race and the rolling surface of the rolling element is in the range of 15 vol. % to 25 vol. %.
According to the third aspect of the present invention, a rolling bearing comprises an inner race, an outer race and a rolling element, in which least one of the inner race, the outer race and the rolling element is obtained by forming from a steel material containing carbon (C) of 0.55 wt % to 1.10 wt %, chromium (Cr) of 2.0 wt % to 9.0 wt % and at least one of molybdenum (Mo) of 1.1 wt % to 4.0 wt %, vanadium (V) of 0.3 wt % to 2.0 wt %, and then subjecting the material thus formed to hardening, tempering and polishing. In the rolling bearing, at least one of a finely particulate molybdenum carbide and a finely particulate vanadium carbide having a particle diameter of 50 nm to 300 nm is distributed in a matrix; and a chromium oxide (Cr2O3) layer having a thickness of 5 nm to 300 nm is provided on at least one of a race track surface of the inner race, a race track surface of the outer race and a rolling surface of the rolling element.
According to a fourth aspect of the present invention, a rolling bearing comprises an inner race, an outer race and a rolling element, in which at least one of the inner race, the outer race and the rolling element is obtained by forming from a steel material containing a carbon (C) of 0.10 wt % to 0.55 wt % and a chromium (Cr) of 5.0 wt % to 14.0 wt %, and then subjecting the material thus formed to carburizing or carbonitriding, hardening, tempering and polishing. In the rolling bearing, said at least one of the inner race, the outer race and the rolling element after carburizing or carbonitriding has a surface carbon concentration of 0.7 wt % to 1.2 wt %; and an amount of austenite retained in a race track surface of the inner race, a race track surface of the outer race and a rolling surface of the rolling element is in the range of 25 vol. % to 50 vol. %; and a chromium oxide (Cr2O3) layer having a thickness of 5 nm to 300 nm is provided on at least one of a race track surface of the inner race, a race track surface of the inner race and a rolling surface of the rolling element.
According to the fifth aspect of the present invention, a rolling bearing comprises an inner race, an outer race and a rolling element, in which at least one of the inner race, the outer race and the rolling element is obtained by forming from a steel material containing carbon (C) of 0.10 wt % to 0.55 wt %, chromium (Cr) of 5.0 wt % to 14.0 wt %, and at least one of molybdenum (Mo) of 0.5 wt % to 4.0 wt % and vanadium (V) of 0.3 wt % to 2.0 wt %, and then subjecting said at least one of the inner race, the outer race and the rolling element thus formed to carburizing or carbonitriding, hardening, tempering and polishing. In the rolling bearing, at least one of a finely particulate molybdenum carbide and a finely particulate vanadium carbide having a particle diameter in the range of 50 nm to 500 nm is distributed in a matrix, said at least one of the inner race, the outer race and the rolling element after carburizing or carbonitriding has a surface carbon concentration of 0.7 wt % to 1.2 wt %; and an amount of austenite retained in a race track surface of the inner race, a race track surface of the outer race and a rolling surface of the rolling element is in the range of 25 vol. % to 50 vol. %; and a chromium oxide (Cr2O3) layer having a thickness of 5 nm to 300 nm is provided on the race track surface of the inner race, the race track surface of the outer race and a rolling surface of the rolling element.
As mentioned above, the rolling bearing according to the first aspect of present invention exhibits a prolonged galling life even in operation under the lubrication at a high traction coefficient.
In particular, the rolling bearing according to the second aspect exhibits a prolonged galling life even in operation under the lubrication at a high traction coefficient with foreign matters mixed therein.
In particular, the rolling bearing according to the third aspect exhibits a prolonged galling life even in operation under the lubrication at a high traction coefficient with water mixed therein.
In particular, the rolling bearing according to the fourth aspect exhibits a prolonged galling life even in operation under the lubrication at a high traction coefficient with foreign matters mixed therein.
In particular, the rolling bearing according to the fifth aspect exhibits a prolonged galling life even in operation under the lubrication at a high traction coefficient with foreign matters and water mixed therein.